1. Field of the Invention
The invention relates to an electric machine and in particular to a position detector for an electric machine. The invention is particularly, although not exclusively, applicable to switched reluctance (SR) machines.
2. Description of Related Art
The general theory of design and operation of switched reluctance machines is well known. It is discussed, for example, in "The Characteristics, Design and Applications of Switched Reluctance Motors and Drives" by Stephenson & Blake, presented at the PCIM '93 Conference and Exhibition at Nurnberg, Germany, Jun. 21-24, 1993, and incorporated by reference herein in its entirety.
The switched reluctance drive is a variable-speed drive system comprising a switched reluctance machine supplied from a power-electronic converter under the control of low-power control electronics. The motor has salient poles on both the stator and the rotor, typically with an excitation coil around each stator pole. These stator coils are grouped to form one or more phase windings. The electrical currents in the windings are typically switched on and off by power-electronic switches. It will be appreciated that the reluctance machine can be operated equally well as a motor or a generator.
FIG. 1 shows a schematic diagram of a typical 3-phase SR machine, which has six stator poles and four rotor poles. The radially opposed poles carrying windings A and A' (phase A) have opposite magnetic polarization determined by the arrangement of the windings. Phases B and C are formed similarly.
The timing of the switching of the currents in the windings is controlled according to the relative angular positions of the stator and rotor poles. This relative position may be detected by a rotor position transducer (RPT) consisting of a rotating member and stationary sensors which supply signals to the control electronics. Those skilled in the art will recognize that there are many known forms of RPT, including, e.g., that shown in European Patent No. 630097, which is incorporated herein by reference.
The RPT is used to ensure that the currents of the phase windings are switched on and off at the appropriate angles of rotation. FIG. 2 illustrates a typical system consisting of a rotating slotted disc and three optical sensors which are switched by the rotation of the disc. The RPT typically consists of a vane on the rotor which interrupts one or more slotted optical switches each comprising a light emitting diode (LED) and a phototransistor. Typically, it is necessary to supply the LED with current of the order of 10-20 mA in order for the slotted switch to operate.
FIG. 3 shows part of a typical converter circuit for a switched reluctance machine. The rectifier bridge 8 supplies the dc link 6 from a single-phase alternating current supply 4. A dc link capacitor 5 acts to smooth the voltage of the dc link.
One known arrangement for supplying current to the LED is also shown in FIG. 3. The arrangement shown is of the "resistive dropper" type commonly used in drives which are designed for cost-sensitive applications such as washing machines, vacuum cleaners, dishwashers etc. A slotted optical switch is shown generally at 10 and includes an LED 12 and a cooperating phototransistor 14. The phototransistor 14 outputs a signal along lines 16 to a control system of any suitable known type to control the switching of the currents in the windings according to the RPT signal. The LED 12 supply current is provided by a resistor, or plurality of resistors 18, connected in series to the DC bus.
In systems designed for 120V.sub.AC utility supplies, the provision of 20 mA of LED current causes a nominal dissipation in the resistors 18 of over 3 W. In systems designed for 230V.sub.AC supplies, the nominal dissipation rises to over 6 W. Provision of a 3 W or 6 W rated resistor uses valuable printed circuit board area and adds to the heat-dissipation requirement of the electronics system. Furthermore, especially in domestic applications, such systems are generally required to operate over a wide supply range, varying between for example 100V.sub.AC and 132V.sub.AC. When the (fixed) resistors 18 are used to derive the LED 12 current, the magnitude of the current can vary significantly from the optimum. In fact it is often necessary to use a conventional power resistor having a significantly higher power rating in order to achieve certification as to the flammability of a product in the domestic appliance market, thus further adding to cost. It will be seen, therefore, that various problems arise in conventional power-supply arrangements for the optical switch 10.